java collections framework - york university...the java collections framework • a coupled set of...

Last Updated: 1/14/10 12:36 PM CSE 2011

Prof. J. Elder - 1 -

Lecture 4.

The Java Collections Framework

Chapters 6.3-6.4

• We will consider the Java Collections Framework as a

good example of how apply the principles of object-

oriented software engineering (see Lecture 1) to the design of classical data structures.

• A coupled set of classes and interfaces that implement

commonly reusable collection data structures.

• Designed and developed primarily by Joshua Bloch

(currently Chief Java Architect at Google).

What is a Collection?

• An object that groups multiple elements into a single

• Sometimes called a container.

What is a Collection Framework?

• A unified architecture for representing and manipulating

collections.

• Includes:

– Interfaces: A hierarchy of ADTs.

– Implementations

– Algorithms: The methods that perform useful computations,

such as searching and sorting, on objects that implement

collection interfaces.

• These algorithms are polymorphic: that is, the same method can be

used on many different implementations of the appropriate

collection interface.

History

• Apart from the Java Collections Framework, the best-

known examples of collections frameworks are the C++

Standard Template Library (STL) and Smalltalk's collection hierarchy.

Benefits

• Reduces programming effort: By providing useful data structures

and algorithms, the Collections Framework frees you to concentrate

on the important parts of your program rather than on the low-level

"plumbing" required to make it work.

• Increases program speed and quality: Provides high-

performance, high-quality implementations of useful data structures

and algorithms.

• Allows interoperability among unrelated APIs: APIs can

interoperate seamlessly, even though they were written

independently.

• Reduces effort to learn and to use new APIs

• Reduces effort to design new APIs

• Fosters software reuse: New data structures that conform to the

standard collection interfaces are by nature reusable.

Core Collection Interfaces

Traversing Collections in Java

• There are two ways to traverse collections:

– using Iterators.

– with the (enhanced) for-each construct

Iterators

• An Iterator is an object that enables you to traverse

through a collection and to remove elements from the

collection selectively, if desired.

• You get an Iterator for a collection by calling its iterator

method.

• Suppose collection is an instance of a Collection.

Then to print out each element on a separate line:

Iterator<E> it = collection.iterator();

while (it.hasNext())

System.out.println(it.next());

Iterators

Iterator interface:

public interface Iterator<E> {

boolean hasNext();

E next();

void remove(); //optional

• hasNext() returns true if the iteration has more elements

• next() returns the next element in the iteration.

• remove() removes the last element that was returned by next.

– remove may be called only once per call to next

– otherwise throws an exception.

– Iterator.remove is the only safe way to modify a collection during iteration

Implementing Iterators

• Could make a copy of the collection.

– Good: could make copy private – no other objects could change

it from under you.

– Bad: construction is O(n).

• Could use the collection itself (the typical choice).

– Good: construction, hasNext and next are all O(1).

– Bad: if another object makes a structural change to the

collection, the results are unspecified.

The Enhanced For-Each Statement

• Suppose collection is an instance of a Collection.

for (Object o : collection)

System.out.println(o);

prints each element of the collection on a separate line.

• This code is just shorthand: it compiles to use

o.iterator().

The Generality of Iterators

• Note that iterators are general in that they apply to any

collection.

– Could represent a sequence, set or map.

– Could be implemented using arrays or linked lists.

ListIterators

• A ListIterator extends Iterator to treat the collection as a list,

allowing

– access to the integer position (index) of elements

– forward and backward traversal

– modification and insertion of elements.

• This is achieved through interfaces for the additional methods:

– hasPrevious()

– previous()

– nextIndex()

– previousIndex()

– set()

– add(e)

Iterator

ListIterator

Iterable

Collection

Abstract

Collection Queue

Abstract

Priority

Queue Array

Abstract

Vector

Linked

Abstract

Sequential List

Interface

Abstract Class

The Java Collections Framework (Ordered Data Types)

The Iterable Interface

• Allows an Iterator to be associated with an object.

• The iterator allows an existing data structure to be

stepped through sequentially, using the following

methods:

– hasNext: does the object have any elements after the current

position?

– next: get the next element

– remove: removes from the sequence the last element returned

by next

Iterable

Collection

Abstract

Collection Queue

Abstract

Priority

Queue Array

Abstract

Vector

Linked

Abstract

Sequential List

Interface

Abstract Class

The Collection Interface

• Allows data to be modeled as a collection of objects. In addition to

the Iterator interface, provides interfaces for:

– Creating the data structure

• add(e)

• addAll(c)

– Querying the data structure

• size()

• isEmpty()

• contains(e)

• containsAll(c)

• toArray()

• equals(e)

– Modifying the data structure

• remove(e)

• removeAll(c)

• retainAll(c)

• clear()

Iterable

Collection

Abstract

Collection Queue

Abstract

Priority

Queue Array

Abstract

Vector

Linked

Abstract

Sequential List

Interface

Abstract Class

The Abstract Collection Class

• Skeletal implementation of the Collection interface.

• For unmodifiable collection, programmer needs to implement:

– iterator (including hasNext and next methods)

– size

• For modifiable collection, need to also implement:

– remove method for iterator

– add

Iterable

Collection

Abstract

Collection Queue

Abstract

Priority

Queue Array

Abstract

Vector

Linked

Abstract

Sequential List

Interface

Abstract Class

The List Interface

• Extends the Collections interface to model the data as an ordered

sequence of elements, indexed by an integer index (position).

• Provides interface for creation of a ListIterator

• Also adds interfaces for:

– Creating the data structure

• add(e)

• add(i, e)

– Querying the data structure

• get(i)

• indexOf(e)

• lastIndexOf

• subList(i1, i2)

– Modifying the data structure

• set(i)

• remove(e)

• remove(i)

Iterable

Collection

Abstract

Collection Queue

Abstract

Priority

Queue Array

Abstract

Vector

Linked

Abstract

Sequential List

Interface

Abstract Class

The Abstract List Class

• Skeletal implementation of the List interface.

• For unmodifiable list, programmer needs to implement methods:

– get

– size

• For modifiable list, need to implement

– set

• For variable-size modifiable list, need to implement

– add

– remove

Iterable

Collection

Abstract

Collection Queue

Abstract

Priority

Queue Array

Abstract

Vector

Linked

Abstract

Sequential List

Interface

Abstract Class

The ArrayList Class

• Random access data store implementation of the List interface

• Uses an array for storage.

• Supports automatic array-resizing

• Adds methods

– trimToSize()

– ensureCapacity(n)

– clone()

– removeRange(i1, i2)

– RangeCheck(i)

– writeObject(s)

– readObject(s)

Iterable

Collection

Abstract

Collection Queue

Abstract

Priority

Queue Array

Abstract

Vector

Linked

Abstract

Sequential List

Interface

Abstract Class

The Vector Class

• Similar to Array List.

• But all methods of Vector are synchronized.

– Guarantees that at most one thread can execute the method at a time.

– Other threads are blocked, and must wait until the current thread completes.

Iterable

Collection

Abstract

Collection Queue

Abstract

Priority

Queue Array

Abstract

Vector

Linked

Abstract

Sequential List

Interface

Abstract Class

The Stack Class

• Represents a last-in, first-out (LIFO) stack of objects.

• Adds 5 methods:

– push()

– pop()

– peek()

– empty()

– search(e)

Iterable

Collection

Abstract

Collection Queue

Abstract

Priority

Queue Array

Abstract

Vector

Linked

Abstract

Sequential List

Interface

Abstract Class

The Abstract Sequential List Class

• Skeletal implementation of the List interface.

• Assumes a sequential access data store (e.g., linked list)

• Programmer needs to implement methods

– listIterator()

– size()

• For unmodifiable list, programmer needs to implement list iterator’s methods:

– hasNext()

– next()

– hasPrevious()

– previous()

– nextIndex()

– previousIndex()

• For modifiable list, need to also implement list iterator’s

– set

• For variable-size modifiable list, need to implement list iterator’s

– add

– remove

Iterable

Collection

Abstract

Collection Queue

Abstract

Priority

Queue Array

Abstract

Vector

Linked

Abstract

Sequential List

Interface

Abstract Class

The Queue Interface

• Designed for holding elements prior to processing

• Typically first-in first-out (FIFO)

• Provides additional insertion, extraction and inspection operations.

• Extends the Collection interface to provide interfaces for:

– offer

– poll

– remove

– peek

– element

Iterable

Collection

Abstract

Collection Queue

Abstract

Priority

Queue Array

Abstract

Vector

Linked

Abstract

Sequential List

Interface

Abstract Class

The LinkedList Class

• Implements the List and Queue interfaces.

• Uses a doubly-linked list data structure.

• Extends the List interface with additional methods:

– getFirst

– getLast

– removeFirst

– removeLast

– addFirst(e)

– addLast(e)

• These make it easier to use the LinkedList class to create

stacks, queues and dequeues (dequeues).

• Not synchronized.

The LinkedList Class

• LinkedList objects are not synchronized.

• However, the LinkedList iterator is fail-fast: if the list is structurally

modified at any time after the iterator is created, in any way except

through the Iterator's own remove or add methods, the iterator will

throw a ConcurrentModificationException.

• Thus the iterator fails quickly and cleanly, rather than risking

arbitrary, non-deterministic behavior at an undetermined time in the

future.

Iterable

Collection

Abstract

Collection Queue

Abstract

Priority

Queue Array

Abstract

Vector

Linked

Abstract

Sequential List

Interface

Abstract Class

The Abstract Queue Class

• Skeletal implementation of the Queue interface.

• Provides implementations for

– add(e)

– remove()

– element

– clear

– addAll(c)

Iterable

Collection

Abstract

Collection Queue

Abstract

Priority

Queue Array

Abstract

Vector

Linked

Abstract

Sequential List

Interface

Abstract Class

The Priority Queue Class

• Based on priority heap

• Provides an iterator

• This will be our next topic!

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